This invention relates to devices that control power output of hydraulic equipment and particularly to such devices that control the output of hydraulic equipment to permit the use of degradable, environmentally-safe, oils for hydraulic fluid.
Although mineral oils have been used as working fluids for hydraulic systems, such as construction equipment, there is a trend, particularly in European nations, to proscribe the use of mineral oils under certain conditions because such oils are environmentally hazardous. Environmentally safe alternatives to mineral oils are available and used in jurisdictions that do not allow the use of mineral oils. For example, biodegradable fluids made from vegetable oil, such as rape (canola) seed oil, are among these few feasible alternatives.
Unfortunately, from a practical standpoint, biodegradable working fluids are inferior substitutes, in some ways, for mineral oils. Such biodegradable fluids are not stable for long periods of time, especially at the high temperatures produced by some hydraulic systems. For example, the usable temperature range of biodegradable working fluid of a rape seed oil type of fluid is from -20.degree. C. to +80.degree. C. while the temperature of the hydraulic fluid in a working hydraulic excavator sometimes exceeds +90.degree. C., despite the use of an oil cooler. Thus it is impractical to use such biodegradable working fluids in such systems. This is particularly true for systems with oil coolers that are sized for use with mineral oils because such systems lack the capacity to maintain the lower fluid temperatures required for continuous use of biodegradable oils.
In countries where use of biodegradable working fluid is required, various measures have been adopted to accommodate the lower temperature requirements of biodegradable oils. One measure is to change the hydraulic fluid before it becomes degraded by increasing the frequency of replacement. Another is to increase the capacity of the oil cooler. Still another is to sound an alarm when excess temperatures are detected and allow the machine to cool down. However, these countermeasures present problems. Increasing the frequency of changes of the working fluid increases the operating cost of the machine. Increasing the capacity of the oil cooler adds initial cost to the hydraulic equipment. Waiting for the equipment to cool down, after excess temperatures are detected, decreases the duty cycle of the equipment and increases operating and labor costs.